The present invention relates to a method for producing a propylene polymer having a very high melt flowability, specifically to a method for producing an isotactic propylene polymer having a dramatically improved melt flowability with a relatively high production yield in convenient way, by improving the reactivity of hydrogen that is served as a molecular weight controlling agent in propylene polymerization.
In propylene polymerization, specifically using methods of gas phase polymerization, slurry polymerization and bulk polymerization, a catalyst system used is generally composed of a Ziegler-type catalyst component, alkylaluminum and an external electron donor. In typical catalyst systems for propylene polymerization as said, the function of an external electron donor is, as is well known, to improve the isotactic index, i.e. stereoregularity of resulted polymer products, by selectively poisoning or converting the active site of non-stereoregularity present on the surface of a solid catalyst. It means the stereoregularity of resulted propylene polymers as well as the polymerization activity and a molecular weight distribution thereof are dependent on the molecular structure of an external electron donor applied. Therefore, in order to improve these properties in a propylene polymer, there have been many conventional techniques which particularly use various silane compounds as an external electron donor.
For example, Japanese laid-open patent publication Nos. Heisei 3-33105 and Heisei 3-33106 disclose a method for preparing polypropylene having high stereoregularity by using triethylaluminum and dialkyldimethoxysilane compound as an external electron donor, alone or as a mixture, together with a solid catalyst component obtained from a carrier which has been prepared by reacting silica and dialkyl magnesium. However, the use of the dialkyldimethoxysilane compound as an external electron donor causes a problem of significant decrease in hydrogen reactivity.
As an another example, U.S. Pat. No. 4,952,649 discloses a method for producing polypropylene having high stereoregularity as much as having an isotactic index of 96-98%, by forming recrystallized particles of a solid catalyst through reacting a solution of magnesium chloride dissolved in 2-ethylhexyl alcohol, with titanium tetrachloride and dialkyl phthalate at −20-130° C., and polymerizing propylene monomers by using the obtained catalyst component together with triethylaluminum as a cocatalyst and various dialkyldialkoxysilanes as an external electron donor in a bulk polymerization process. Additionally, U.S. Pat. No. 6,384,160 discloses a method for producing polypropylene with an isotactic index of 92-98% by using a solid catalyst component which is obtained by reacting diethoxy magnesium with titanium tetrachloride and an internal electron donor, diisobutyl phthalate or diethyl phthalate, together with a cocatalyst, triethylaluminum and an external electron donor, dialkyldimethoxysilane.
However, when using the main catalyst components and the external electron donors disclosed in those above-mentioned U.S. patents, the reactivity of a molecular weight controlling agent such as hydrogen is not sufficient enough to obtain polypropylene having a melt flow rate of 50 g/10 minutes or more in practical. That is, when feeding a large amount of hydrogen into a polymerization reactor for compensating the insufficient hydrogen reactivity in a conventionally practiced process, there is a risk of explosion and the like owing to the designed pressure limit of the reactor, therefore the possible feeding amount of hydrogen is practically limited. By that reason, when using the main catalyst components and the external electron donors disclosed in the above-mentioned US patents, it is practically impossible to increase the hydrogen pressure to the extent that it is required for producing polypropylene having a high melt flow rate.
Additionally, a method of olefin polymerization which uses phenyltriethoxysilane as an external electron donor, disclosed in Japanese laid-open patent publication No. Showa 57-63311, has a problem of toxicity owing to phenyl groups released from the external electron donor after the polymerization, and a method of olefin polymerization which uses dialkyldimethoxysilane as an external electron donor, disclosed in Japanese laid-open patent publication No. Heisei 2-170803, cannot provide a polymer having a high melt index. Further, a method of olefin polymerization disclosed in Japanese laid-open patent publication No. Heisei 8-157520, which uses alkylalkoxysilane substituted with cycloalkyl groups as an internal electron donor, may provide a polymer having a rather high melt flow rate, however, it has a problem of relatively too low catalyst activity.